Ion implantation is a process of depositing chemical species into a substrate by direct bombardment of the substrate with energized ions. In semiconductor manufacturing, ion implanters are used primarily for doping processes that alter the type and level of conductivity of target materials. A precise doping profile in an integrated circuit (IC) substrate and its thin-film structure is often crucial for proper IC performance. To achieve a desired doping profile, one or more ion species may be implanted in different doses and at different energy levels.
FIG. 1 depicts a conventional ion implanter system 100. The ion implanter system 100 may comprise an ion source 102 and a complex series of components through which an ion beam 10 passes. The series of components may include, for example, an extraction manipulator 104, a filter magnet 106, an acceleration or deceleration column 108, an analyzer magnet 110, a rotating mass slit 112, a scanner 114, and a corrector magnet 116. Much like a series of optical lenses that manipulate a light beam, the ion implanter components can filter and focus the ion beam 10 before steering it towards a target wafer 120 (located in a wafer plane 12). A number of measurement devices, such as a dose control Faraday cup 118, a traveling Faraday cup 124, and a setup Faraday cup 122, may be used to monitor and control the ion beam conditions.
The ion source 102 is a critical component of the ion implanter system 100. The ion source 102 is required to generate a stable and reliable ion beam 10 for a variety of different ion species and extraction voltages.
FIG. 2 depicts a typical embodiment of an ion source 200 that may be used in the ion implanter system 100. The ion source 200 may be an inductively heated cathode (IHC) ion source, which is typically used in high current ion implantation equipment. The ion source 200 comprises an arc chamber 202 with conductive chamber walls 214. At one end of the arc chamber 202 there is a cathode 206 having a tungsten filament 204 located therein. The tungsten filament 204 is coupled to a first power supply 208 capable of supplying a high current. The high current may heat the tungsten filament 204 to cause thermionic emission of electrons. A second power supply 210 may bias the cathode 206 at a much higher potential than the tungsten filament 204 to cause the emitted electrons to accelerate towards and heat the cathode 206. Heating the cathode 206 causes the cathode 206 to emit thermionic electrons into the arc chamber 202. A third power supply 212 may bias the chamber walls 214 with respect to the cathode 206 so that the electrons are accelerated at a high energy into the arc chamber. A source magnet (not shown) may create a magnetic field B inside the arc chamber 202 to confine the energetic electrons, and a repeller 216 at the other end of the arc chamber 202 may be biased at a same or similar potential as the cathode 206 to repel the energetic electrons. A gas source 218 may supply a precursor implant species (e.g., AsH4, PH4, BF3, GeF4) into the arc chamber 202. The energetic electrons may interact with the precursor implant species to produce a plasma 20. An extraction electrode (not shown) may then extract ions 22 from the plasma 20 through an extraction aperture 220 for use in the ion implanter 100.
A problem that currently exists in conventional ion implantation is that ion implanters that are typically designed for efficient operation at relatively high implant energies (e.g., 10's kV) may not function efficiently at lower energies (e.g., sub-kV). One application, for example, which utilizes low-energy dopant beams is the formation of ultra-shallow transistor junctions in CMOS (complimentary metal-oxide semiconductor) manufacturing. Specifically, implantation of low-energy boron, a p-type dopant, into silicon wafers is particularly important.
This inefficiency at lower energies is generally caused by space charge limitations. The Child-Langmuir relation, J˜(Z/A)1/2V3/2/d2, describes the current density limit of a space charge dominated beam. Here, an ion beam's current density limit J scales proportionally to the square root of a charge to mass ratio (Z/A) and to the three-halves power of extraction potential V. Thus, at lower beam energies (e.g., lower extraction potential V), less ion current density may be obtained when compared to extraction of the same species at higher energies.
For example, space charge in low energy beams tends to cause the beam cross-section area (“profile”) to grow larger as ions proceed along the beam line (“beam blow-up”). When the beam profile exceeds the profile for which the implanter's transport optics have been designed, beam loss through vignetting occurs. For instance, at 500 eV transport energy, conventional ion implanters typically cannot transport adequate boron beam current for efficient commercial manufacturing. Wafer throughput is reduced because of low implantation dose rate.
One method to overcome the space charge limitation of low energy beams is to implant the required dopant in its molecular form. For example, U.S. patent application Ser. Nos. 11/504,355 and 11/342,183, which are hereby incorporated by reference in their entirety, describe a method of molecular ion implantation and specifically implantation of C2B10Hx ions generated from C2B10H12 (carborane). Molecular ion implantation allows the use of higher extraction energies to achieve the equivalent implant depth as obtained from a low energy atomic ion implant. The relationship between the higher molecular extraction energy and the equivalent atomic implant depth may be expressed by:Emolecule=(molecule mass/atomic mass)×Eequivalent,where Emolecule is the energy of the molecular ion and Eequivalent is the implant energy required to achieve the desired implant depth using an atomic ion only. For example, instead of implanting an ion current I of atomic B+ ions at an energy E, a carborane molecular ion (C2B10H12+) for example, may be implanted at an energy approximately 13×E and at an ion current of 0.10×I. Although the resulting implantation depth and dopant concentration (dose) of both approaches have been shown to be generally equivalent, the molecular implantation technique may have significant potential advantages. For example, since the transport energy of the carborane ion is thirteen times that of a dose-equivalent boron ion and the carborane current is one-tenth that of the boron current, space charge forces may be substantially reduced when compared to monatomic boron implantation.
However, conventional ion implanters utilizing an IHC ion source 102 are relatively inefficient at producing large molecular ions. For example, while BF3 gas is typically used by conventional ion sources to generate B+ ions, carborane (C2B10H14) must be used to generate the carborane ion C2B10Hx+. Carborane is a solid material which has a significant vapor pressure, on the order of 1 Torr at 20° C., melts at 100° C., and decomposes at 750° C. For efficient production of C2B10Hx+, carborane must be vaporized below 100° C. and must operate in an ion source whose local environment (e.g., chamber wall and chamber components) is below 750° C. to avoid decomposition of the majority of the vaporized molecules. However, the operating temperatures of IHC sources are typically 800° C. to 2300° C. from the chamber body to the heated button cathode. While these temperatures are required for the thermal dissociation of BF3 and subsequent B+ ion creation, they are not conducive for the production carborane ions that can be readily dissociated by the high operating temperatures. In addition, such extremely high temperatures shorten the performance and lifetime of IHC ion sources. As a result, the performance degradation and short lifetime of IHC ion sources greatly reduce the productivity of ion implanters.
Furthermore, ion source vaporizers typically cannot operate reliably at the lower temperatures required for carborane, due to radiative heating from the ion source to the vaporizer. For example, vaporizer feed lines may easily become clogged with deposits from decomposed vapor as the carborane vapor interacts with their hot surfaces. Hence, conventional ion sources are incompatible with carborane ion implantation.
In view of the foregoing, it may be understood that there are significant problems and shortcomings associated with current ion source technologies.